Spiders, again, barely
undergo any metamorphosis. The larvae of most insects pass through a
worm-like stage, whether they are active and adapted to diversified habits,
or are inactive from being placed in the midst of proper nutriment, or from
being fed by their parents; but in some few cases, as in that of Aphis, if
we look to the admirable drawings of the development of this insect, by
Professor Huxley, we see hardly any trace of the vermiform stage.
Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Muller has made the remarkable discovery that certain shrimp-like
crustaceans (allied to Penoeus) first appear under the simple nauplius-
form, and after passing through two or more zoea-stages, and then through
the mysis-stage, finally acquire their mature structure: now in the whole
great malacostracan order, to which these crustaceans belong, no other
member is as yet known to be first developed under the nauplius-form,
though many appear as zoeas; nevertheless Muller assigns reasons for his
belief, that if there had been no suppression of development, all these
crustaceans would have appeared as nauplii.
How, then, can we explain these several facts in embryology--namely, the
very general, though not universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes, being
at an early period of growth alike; the common, but not invariable,
resemblance between the embryos or larvae of the most distinct species in
the same class; the embryo often retaining, while within the egg or womb,
structures which are of no service to it, either at that or at a later
period of life; on the other hand, larvae which have to provide for their
own wants, being perfectly adapted to the surrounding conditions; and
lastly, the fact of certain larvae standing higher in the scale of
organisation than the mature animal into which they are developed? I
believe that all these facts can be explained as follows.
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